Automobiles of FR drive model or four-wheel drive model generally comprise, as a power transmission transmitting the rotational driving force of the engine to drive wheels, a rotational power transmission mechanism that transmits the rotational driving force from the transmission (rotational drive member), which outputs the power of the engine with speed ratio changes, through a propeller shaft (rotational power transmission member) to an axle device (rotational driven member). For example, the engine is placed in the front part of the vehicle, and the output rotation of the transmission, which is coupled to the engine, is transmitted through the propeller shaft to the rear axle device. Various such power transmissions have been known, and some are disclosed in Japanese Laid-Open Patent Publication No. H10(1998)-194004 and in Japanese Laid-Open Utility-Model Publication No. H07(1995)-8632.
The propeller shaft comprises a hollow pipe member and yoke members, which are attached at the respective ends of the pipe member, each yoke member constituting a universal joint. Through the universal joints, the output shaft of the transmission and the input shaft of the rear axle are connected to the propeller shaft. The hollow pipe member, which has been conventionally made of an iron material, are nowadays made of Fiber Reinforced Plastic (FRP) or Carbon Fiber Reinforced Plastic (CFRP), which is a light weight material that provides sufficient strength. A propeller shaft comprising a hollow pipe member made of such a material as FRP or CFRP is hereinafter referred to as an FRP propeller shaft.
By the way, to increase the safety of an automobile in an event of head-on collision, the vehicle has a structure that allows the front part of the engine room to collapse for absorbing the impact energy at the time of head-on collision (this structure is a so-called crushable construction). Accordingly, the engine in the engine room and the transmission, which is coupled to the engine as a one-piece body, are designed to recede at the time of head-on collision. In this case, if the vehicle comprises a rotational power transmission mechanism, which transmits the rotational driving force from the transmission through the propeller shaft to the rear axle device as mentioned above, then the propeller shaft may interfere with the recession of the engine and the transmission. Especially, if the propeller shaft is an FRP propeller shaft, which has a high rigidity against a compressive force in its axial direction and therefore shows a property of low impact energy absorption, then the FRP propeller shaft can obstruct an appropriate recessive movement of the engine, etc., which will result in a low performance of impact energy absorption. Therefore, for example, Japanese Laid-Open Patent Publication No. H 11(1999)-78563, Patent publication No. 3063583, Japanese Laid-Open Patent Publication No. H7(1995)-91431 and Japanese Laid-Open Patent Publication No. H9(1997)-175202 disclose device arrangements for improving the impact energy absorption property, in which arrangements the FRP shaft deforms and breaks up when it receives a comparatively small axial compressive force.